Two metallic spheres, Q1 and Q2, which constitute
a capacitor are connected to metallic rods, M and
N, which behave as an inductor with a spark gap
S between them. A large potential difference is
obtained with the help of induction coil to produce
spark in the spark gap. Such an arrangement can
be considered as an L-C oscillator circuit and is
also known as a Hertzian dipole. At any instant
when Q1 has a positive charge, Q2 has the same
amount of negative charge. The polarity on the
spheres Q1 and Q2 keep changing with a definite time period with change passing through the spark gap.
A second spark gap, R, is arranged to detect the emission of the electromagnetic waves. Q1 and Q2 are arranged by sliding them on the rod to produce spark in spark gap R due to resonance. Suppose the spheres are charged as shown in the figure, at any instant of time.
The electric intensity at points C on the perpendicular bisector of Q1 Q2 are shown by a and b due to the charged spheres Q1 and Q2 respectively, the resultant of which is E parallelto MN as shown in the figure. Similarly, the electric field intensity at D is also parallel to MN but is of the smaller
magnitude. Thus there is a gradual decrease in the intensity of the electric field at a given instant as we move away from MN.
As the spark is produced in the spark gap, electrons flow from the sphere Q1 to Q2 reducing negative charge on Q1 and positive charge on Q2. With one half cycle of time elapsing, the charge on Q1 becomes positive and that on Q2 negative. Now the electric fields at C and D are in opposite directions.
Such periodic sparking results in vertical oscillations of electrons which in turn produces an oscillating electric field in space. Also the oscillations of the electrons give rise to a periodically changing electric current. This produces a periodically oscillating magnetic field at points such as C and D, the direction of which is perpendicular to that of the electric field as can be known using Ampere’s right hand rule.
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